1. Field of the Invention
The present invention relates to an optical scanning apparatus and an image forming apparatus using the same. In particular, the present invention is suitable for an image forming apparatus, which employs an electrophotographic process, such as a laser beam printer, a digital copying machine or a multi-function printer. In this case, in the image forming apparatus, after a light beam emitted from light source means is made incident on a deflecting surface of an optical deflector to be deflected so as to make its width wider than that of the deflecting surface in a main scanning direction, a surface to be scanned is scanned with the resultant light beam through a scanning optical system having fθ characteristic to record image information on the surface to be scanned.
2. Related Background Art
Heretofore, an optical scanning apparatus has been widely used in an image forming apparatus, which employs an electrophotographic process, such as a laser beam printer, a digital copying machine or a multi-function printer.
In this optical scanning apparatus, a light beam emitted from light source means constituted by a semiconductor laser or the like, for example, is converted into a nearly collimated light beam (or a convergent light beam or a divergent light beam) by a collimator lens to be guided to a deflecting surface (deflecting/reflecting surface) of an optical deflector including a polygon mirror (rotational polygon mirror). Thus, a surface to be scanned is scanned with the light beam deflected by the optical deflector at a constant speed while the light beam is imaged in a spot-like shape on the surface to be scanned using a scanning optical system (fθ lens system).
In addition, in this optical scanning apparatus, in a sub-scanning cross section (or cross-section) orthogonal to a main scanning cross section, there may be used in some cases a so-called optical face tangle error correction optical system with which a nearly collimated light beam emitted through a collimator lens is condensed in the vicinity of a deflecting surface of an optical deflector using a cylindrical lens having a predetermined refractive power only in a sub-scanning direction, and is then re-imaged on a surface to be scanned by a scanning optical system.
On the other hand, there is used an under filled scanning optical system (hereinafter also referred to as “UFS”) in which the whole light beam is reflected/deflected by a deflecting surface in a state in which a width of a nearly collimated light beam emitted through a collimator lens is made narrower than that of the deflecting surface in a main scanning cross section. Though the UFS can deflect and scan the light most efficiently, the deflecting surface needs to be made large to some degree. As a result, the UFS has a tendency to scale up the optical deflector.
In recent years, for the laser beam printer, the digital copying machine or the multi-function printer, a demand for increasing the printing speed has increased. However, in order to increase the printing speed, it is necessary to rotate the optical deflector at a high speed or to increase the number of deflecting surfaces of the deflector. In the case of the UFS having the large optical deflector, if the optical deflector is rotated at a higher speed, there arises a problem concerning heat generation, noises or power consumption of the optical deflector. In addition, if the number of deflecting surfaces of the deflector is increased, the optical deflector is further scaled up. As a result, likewise, there arises a problem concerning heat generation, noises or power consumption of the optical deflector.
On the other hand, in case of an over field scanning optical system (hereinafter also referred to as “OFS”) in which a light beam emitted from light source means is made incident on a deflecting surface of an optical deflector with its width wider than that of the deflecting surface in a main scanning direction, the optical deflector itself is smaller than that included in the UFS. As a result, the optical deflector can be rotated at a higher speed, and the number of reflecting surfaces can be increased.
In the OFS, a light beam is made incident on a deflecting surface of an optical deflector with its width wider than that of the deflecting surface in a main scanning direction, and a part of the light beam is cut away by the deflecting surface. Hence, there is encountered a problem that the higher output power is required for the light source means all the more because the utilization efficiency of the light energy is lower than that in the UFS. However, in recent years, a high-power semiconductor laser has been developed as the light source means, and hence this problem has been solved.
However, the above-mentioned OFS involves the following problems.
In the OFS, a light beam emitted from light source means is made incident on a deflecting surface of an optical deflector with its width wider than that of the deflecting surface of the optical deflector in a main scanning direction, and is then deflected so as to be cut away by the deflecting surface of the optical deflector to be guided to a surface to be scanned. Thus, different portions of the light beam made incident on the optical deflector are used in correspondence to an image height of the surface to be scanned. For example, for the light beam guided to a central portion on the surface to be scanned, a central portion of the light beam made incident on the optical deflector is used, while for the light beam guided to a peripheral portion of the surface to be scanned, a peripheral portion of the light beam made incident on the optical deflector is used. For this reason, when the light beam made incident on the optical deflector has a difference in wavefront between the central portion (the vicinity of an on-axis portion) and the peripheral portion of the light beam as typified by spherical aberration, for example, a so-called field curvature is caused in which a difference occurs in imaging positions on the surface to be scanned in correspondence to an image height.
A beam spot size is enlarged on the surface to be scanned in correspondence to the image height due to the field curvature. In addition, the OFS is more likely to cause a side lobe in a spot profile on the surface to be scanned due to the field curvature as compared with the UFS.
The problem such as the enlargement of the spot size, and the side lobe in the spot profile results in a negative effect such as reduction in the resolution and thickening a fine line on an image recorded on the surface to be scanned. Since this tendency becomes more remarkable as the beam spot diameter is small, it has been especially a serious problem in coping with the high image quality of an image forming apparatus.
Various optical scanning apparatuses each using the OFS with which the above-mentioned problems are solved have been proposed (refer to U.S. Pat. No. 5,757,535, Japanese Patent Application Laid-open No. 2001-59946, and Japanese Patent Application Laid-open No. 2002-267976, for example).
U.S. Pat. No. 5,757,535 discloses an optical scanning apparatus including: light source means; a first optical system for converting a light beam diverged in at least a main scanning direction from the light source means into a nearly collimated flux; a rotational polygon mirror having a plurality of deflecting surfaces (reflecting surfaces) parallel to a rotation axis, the rotational polygon mirror serving to rotate around the rotation axis at a nearly constant angular velocity to deflect/reflect an incident light beam along a predetermined main scanning direction by the plurality of deflecting surfaces; and a second optical system for converging the deflected/reflected light beam onto a surface to be scanned so that the surface to be scanned is scanned with the light beam deflected/reflected by the rotational polygon mirror along a main scanning direction at a nearly constant speed, in which the optical scanning apparatus is of an over field type in which the light beam from the light source means is made incident on the plurality of deflecting surfaces of the rotational polygon mirror so as the light beam to straddle the plurality of deflecting surfaces thereof, and a wavefront for correcting aberration generated in the second optical system to which a part of the light beam deflected/reflected by one deflecting surface of the rotational polygon mirror is made incident is formed by the first optical system on which the whole light beam from the light source means is made incident. That is, U.S. Pat. No. 5,757,535 discloses that the spherical aberration generated in the first optical system as an incidence optical system from the light source means to the optical deflector and the field curvature caused in the second optical system as a scanning optical system from the optical deflector to the surface to be scanned are made cancel each other.
Japanese Patent Application Laid-open No. 2001-59946 discloses an optical scanning apparatus including: a first optical system having light source means for converting a light beam emitted from the light source means into a nearly collimated light beam in a main scanning cross section; a second optical system including the first optical system for allowing the light beam made incident on a deflecting surface of an optical deflector with its width wider than that of the deflecting surface in a main scanning direction; and a third optical system for imaging the light beam deflected/reflected by the optical deflector on a surface to be scanned, in which the first optical system includes an achromatic lens having a negative lens and a positive lens, or a single lens having at least one aspherical surface thereof. That is, the first optical system as a condenser lens (unit) for condensing the light beam from the light source means adopts two lenses, i.e., a concave lens and a convex lens, or an aspherical lens, thereby suppressing generation of the spherical aberration itself in the first optical system.
Japanese Patent Application Laid-open No. 2002-267976 discloses that at least one resin lens and a glass lens are provided between a coupling lens and an optical deflector, and a shape of the resin lens within a main scanning cross section is made non-arcuate (aspherical surface), thereby correcting the spherical aberration.
However, in U.S. Pat. No. 5,757,535, since the first optical system (incidence optical system) and the second optical system (scanning optical system) are designed so as to be paired with each other, in a case where when the first and second optical systems are diverted to another imaging apparatus, the specification, e.g., the scanning speed, is different from original one, if the first optical system is redesigned, the second optical system also needs to be redesigned accordingly.
In Japanese Patent Application Laid-open No. 2001-59946 in which that point is improved, since two condenser lenses are used or the aspherical lens is used, this configuration is likely to involve a disadvantage in terms of the cost. In particular, when the optical stability against the heat is taken into consideration, the resin lens is hard to be used as the condenser lens disposed in the vicinity of the laser light source as a heat source. Thus, since the condenser lens is necessarily made of glass, the cost-up due to an increase in the number of lenses or utilization of the aspherical surface cannot be disregarded.
Japanese Patent Application Laid-open No. 2002-267976 discloses that at least one resin lens and the glass lens are provided between the coupling lens and the optical deflector, and the shape of the resin lens within a main scanning cross section is made non-arcuate (aspherical surface), thereby correcting the spherical aberration. However, a method of solving the problem that the spherical aberration which is generated in the incidence optical system and which is inherent in the OFS causes the field curvature in the optical scanning apparatus is not described at all.